5-2-1-1 SiC Crystallography
Silicon carbide occurs in many different crystal structures, called polytypes. Despite the fact that all SiC polytypes chemically consist of 50% carbon atoms covalently bonded with 50% silicon atoms, each SiC polytype has its own distinct set of electrical semiconductor properties. While there are over 100 known polytypes of SiC, only a few are commonly grown in a reproducible form acceptable for use as an electronic semiconductor. The most common polytypes of SiC presently being developed for electronics are 3C-SiC, 4H-SiC, and 6H-SiC. The atomic crystal structure of the two most common polytypes is shown in the schematic cross section in Figure. As discussed much more thoroughly in References 9 and 10, the different polytypes of SiC are actually composed of different stacking sequences of Si–C bilayers (also called Si–C double layers), where each single Si–C bilayer is denoted by the dotted boxes in Figure. Each atom within a bilayer has three covalent chemical bonds with other atoms in the same (its own) bilayer, and only one bond to an atom in an adjacent bilayer. Figure 5.1a shows the bilayer of the stacking sequence of 4H-SiC polytype, which requires four Si–C bilayers to define the unit cell repeat distance along the c-axis stacking direction (denoted byMiller indices). Similarly,the 6H-SiC polytype repeats its stacking sequence every six bilayers throughout the crystal along the stacking direction.The direction depicted in Figure is often referred to as one of (along with ) the a-axis directions. SiC is a polar semiconductor across the c-axis, in that one surface normal to the c-axis is terminated with silicon atoms while the opposite normal c-axis surface is terminated with carbon atoms. As shown, these surfaces are typically referred to as “silicon face” and “carbon face” surfaces, respectively. Atoms along the left-or right-side edge of Figure would reside on “a-face” crystal surface plane normal to the direction. 3C-SiC, also referred to as β-SiC, is the only form of SiC with a cubic crystal lattice structure. The noncubic polytypes of SiC are sometimes ambiguously referred to as α-SiC. 4H-SiC and 6H-SiC are only two of the many possible SiC polytypes with hexagonal crystal structure. Similarly, 15R-SiC is the most common of the many possible SiC polytypes with a rhombohedral crystal structure.